The objective of this research is to investigate the mechanism of control of contraction in striated muscle at the cellular and molecular level. The major mechanism of control is probably through regulation of intracellular calcium concentration but many factors such as (Mg), (MgATP), pH, etc. vary the relationship between tension and calcium concentration. In addition to these chemical factors, mechanical factors such as sarcomere length are also important. The effects of these other factors on control of contraction will be investigated and estimates made of their importance in controlling contraction of normal muscle. This present work will involve mechanical measurements on chemically stimulated "skinned" skeletal and cardiac muscle fibers where the composition of the sarcoplasm can be controlled, and mechanical measurements on whole muscle fibers of the frog and the giant barnacle.